I tried to reproduce the described problem using the provided code, but to no avail. Please compare the attached test sample with your actual project and let me know if I have missed something. Note that I replaced the RadGrid with a button control as in this scenario the grid is used for triggering the callback and the effect will be the same if a button is used for this purpose. Here
you can find a screen capture of the behavior on my end.
Such a problem can be encountered when ASP.NET client callbacks are used and controls are added dynamically to the page in events which are not run during the client callback. In a client callback the page runs in a modified page life cycle - the page is initiated and its controls and other members are created. If the RadXmlHttpPanel is dynamically added on the page, it will not be re-created when the XmlHttpPanel's client callback is in process and the target control for the client callback (i.e. the RadXmlHttpPanel) could not be found because it is not present (it wasn't recreated) on the page.
If you are still having difficulties, please open a support ticket
and send a modified version of the attached sample so that the issue can be reproduced. In case you are not able to achieve this, you can also provide a simple, fully runnable page
that isolates the problematic behavior so that I can inspect it locally and suggest an according solution.
the Telerik team
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